Structure of NOx sensor and its calculating method of total total NOx concentration

ABSTRACT

The present invention relates to a NO x  sensor and a calculating method of total NO x  concentration using the same, and more particularly, to a NO x  sensor which has improved sensitivity to NO and NO 2  and simply calculates NO, NO 2  and total NO x  concentration, and a calculating method of total NO x  concentration using the same. 
     The NO x  sensor of the present invention comprises an oxygen ion conductive solid electrolyte  10 ; an oxide sensing electrode  20  formed at the oxygen ion conductive solid electrolyte  10 ; a noble metal electrode  30 ; and a lead line  40  connected to each of the oxygen ion conductive solid electrolyte  10  or the oxide sensing electrode  20  or the noble metal electrode  30 , wherein the oxygen ion conductive solid electrolyte  10  and the oxide sensing electrode  20  form at least two interfaces. 
     Further, the calculating method of total NO x  concentration of the present invention comprises the steps of a) measuring voltages or currents of each of two or more NO x  sensors  100  as described above; b) substituting the measured voltages or currents into a series of NO x  concentration calculating formulars of each of the NO x  sensors  100  so as to calculate NO and NO 2  concentrations separately; and c) adding the NO and NO 2  concentrations so as to calculate the total NO x  concentration. 
     According to the NO x  sensor and the calculating method of measuring total NO x  concentration using the same, it is possible to prevent the deterioration of the sensitivity even in an atmosphere that the NO accounts for a major percent, thereby increasing the measurement precision. And by using a simple method in which the electromotive force measured in two or more sensors or two or more interfaces is substituted into the NO x  concentration calculating formular, it is possible to facilely calculate the NO, NO 2  and NO x  concentration.

TECHNICAL FIELD

The present invention relates to a NO_(x) sensor and a calculatingmethod of total NO_(x) concentration using the same, and moreparticularly, to a NO_(x) sensor which has improved sensitivity to NOand NO₂ and simply calculates NO, NO₂ and total NO_(x) concentration,and a calculating method of total NO_(x) concentration using the same.

BACKGROUND ART

When nitrogen contained in combustion air and fuel is combined withoxygen by influence of temperature and the like, nitrogen oxide isgenerated, and the nitrogen oxide including NO, NO₂ and N₂O₃ isexpressed as NO_(x).

Particularly, the NO₂ is a maroon colored poisonous gas having a pungentsmell. The NO and NO₂ accounting for a major percent of the whole NO_(x)are mainly generated from a transportation means and becomes a cause ofair pollution. Therefore, there is an increased necessity of measuringNO_(x) concentration.

As a conventional NO_(x) sensor for measuring the NO_(x) concentration,there has been proposed a mixed potential type NO_(x) sensor as shown inFIG. 1 a.

Referring to FIG. 1, the mixed potential type NO_(x) sensor includes anoxygen ion conductor 110 using stabilized zirconia; an oxide sensingelectrode 120 formed at one side of the oxygen ion conductor 110; anoble metal electrode 130; and a noble metal reference electrode 140.And the mixed potential type NO_(x) sensor is characterized by measuringan electromovite force generated between both ends of the noble metalelectrode 130 and the noble metal reference electrode 140.

In the mixed potential type NO_(x) sensor, since the oxide sensingelectrode 120 has reactivity to NO_(x) and oxygen, but the referenceelectrode 140 has reactivity only to oxygen, an electromovite forcegenerated between the reference electrode 140 and the oxide sensingelectrode 120 is occurred according to the NO_(x) concentrationcontained in gas. And the difference of electromotive force is measuredand thus a NO_(x) amount is measured.

FIG. 1 b is a graph showing the electromotive force of the NO_(x) sensorat 700° C. and an oxygen partial pressure of 5% in case that it isexposed to a gas where NO or NO₂ accounts for a major percent.

As shown in FIG. 1 b, the NO₂ accounts for a major percent before 80minutes, and the NO accounts for a major percent after 80 minutes. Incase that the NO₂ is the major component of the gas, the measuredelectromotive force has a similar shape with NO₂ concentration. However,in case that the NO is a major composition, it can be understood thatthe NO_(x) sensor can not normally carry out its function due toremarkable reduction of its sensibility caused by the NO.

That is, in the mixed potential type NO_(x) sensor, if the NO₂ isexisted in the gas, reactions take place according to the followingformulas (1) and (2), and if the NO is existed, reactions take placeaccording to the following formulas (3) and (4):

$\begin{matrix}\begin{matrix}{{In}\mspace{14mu} {case}\mspace{14mu} {of}\mspace{14mu} {NO}_{2}\text{:}} & \left. {{NO}_{2} + {2e^{-}}}\rightarrow{{NO} + O^{2 -}} \right. \\\; & \left. O^{2 -}\rightarrow{{{1/2}\mspace{11mu} O_{2}} + {2e^{-}}} \right.\end{matrix} & \begin{matrix}(1) \\(2)\end{matrix} \\\begin{matrix}{{In}\mspace{14mu} {case}\mspace{14mu} {of}\mspace{14mu} {NO}\text{:}} & \left. {{NO} + O^{2 -}}\rightarrow{{NO}_{2} + {2e^{-}}} \right. \\\; & \left. {{{1/2}\mspace{11mu} O_{2}} + {2e^{-}}}\rightarrow O^{2 -} \right.\end{matrix} & \begin{matrix}(3) \\(4)\end{matrix}\end{matrix}$

As represented by the formulas (1) to (4), a sign of the electromotiveforce generated between the noble metal electrode like Pt or gold andthe oxide sensing electrode in case of that the NO is present iscontrary to that of the electromotive force generated between the noblemetal electrode and the oxide sensing electrode in case of that the NO₂is present. Therefore, in case that the NO and NO₂ are mixed togetherlike in the automobile gas atmosphere, the NO_(x) sensor using a mixedpotential type has a disadvantage that it is difficult to measure thetotal NO_(x) concentration due to the characteristic that theelectromotive forces tend to move in opposite direction for NO and NO₂exposure.

FIG. 1 c is a graph showing the electromotive force in case that NiO asthe sensing electrode is formed in the NO_(x) sensor, and FIG. 1 d is agraph showing the electromotive force in case that CuO as the sensingelectrode is formed in the NO_(x) sensor. With reference to FIGS. 1 cand 1 d, it can be understood that accuracy of the NO_(x) sensor isdeteriorated due to reduction of the electromotive force caused byincrease of NO, although the NO₂ concentration is constantly maintainedor increased. It means that the NO_(x) sensor simply using the mixedpotential can not be used in the mixed gas of the NO and NO₂.

In actuality, in case that the NiO is used as the sensing electrode, asshown in FIG. 1 c, if the NO concentration is changed from 10 ppm to 100ppm, a change in the electromotive force is −6.5 mV, and if the NO₂concentration is changed from 10 ppm to 100 ppm, a change in theelectromotive force is 83.7 mV. As shown in FIG. 1 d, in case that theCuO is used as the sensing electrode, if the NO concentration is changedfrom 10 ppm to 100 ppm, a change in the electromotive force is −3.4 mV,and if the NO₂ concentration is changed from 10 ppm to 100 ppm, a changein the electromotive force is 66.0 mV.

As described above in terms of their sensitivities, there is a problemthat, although the NO has a high concentration, the conventional mixedpotential type NO_(x) sensor can not sense it in the presence of NO₂.

To solve the above problem, there has been proposed a calculating methodof total NO_(x) concentration, in which a conversion cell 150 having amulti-layer structure is provided at an inlet port through whichmeasurement gas is introduced so that the NO_(x) is converted to asingle component and thus unified into the NO or the NO₂ so as tomeasure the total NO_(x) concentration, as shown in FIG. 2.

In the above-mentioned method, the unifying process for converting NO₂to NO or converting NO to NO₂ should be performed. However, since theconversion cell 150 has a limit to convert the mixture gases to the NOor NO₂ completely, it is difficult to precisely measure the total NO_(x)concentration in the mixture gas.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a NO_(x) sensorwhich can separately measure the NO and the NO₂ concentration, and alsowhich can improve reactivity of the NO in an atmosphere that the NOaccounts for a major percent so as to precisely measure the NOconcentration.

It is another object of the present invention to provide a calculatingmethod of total NO_(x) concentration using the same, which can facilelycalculate the total NO_(x) concentration by adding the separatelymeasured NO and NO₂ concentrations which are obtaind from NO_(x)concentration calculating formulars.

To achieve the objects, the present invention provides NO_(x) sensor,comprising an oxygen ion conductive solid electrolyte; an oxide sensingelectrode formed at the oxygen ion conductive solid electrolyte; a noblemetal electrode; and a lead line connected to each of the oxygen ionconductive solid electrolyte or the oxide sensing electrode or the noblemetal electrode, wherein the oxygen ion conductive solid electrolyte andthe oxide sensing electrode form at least two interfaces.

Preferably, in the NO_(x) sensor, NO_(x) concentration is calculated byvoltages measured after a constant currents is applied between the twointerfaces of the oxide sensing electrodes 20 or by currents measuredafter a constant voltages is applied between the two interfaces of theoxide sensing electrodes 20.

Preferably, at least two or more oxide sensing electrodes 20 are formedon a surface of the oxygen ion conductive solid electrolyte 10, or theoxide sensing electrode 20 is formed on upper and lower surfaces of theoxygen ion conductive solid electrolyte 10.

Preferably, two or more oxygen ion conductive solid electrolytes 10 areformed to be apart from each other at a predetermined distance, and theoxide sensing electrode 20 is interposed between the oxygen ionconductive solid electrolytes 10, or two or more oxide sensingelectrodes 20 are formed to be apart from each other at a predetermineddistance, and the oxygen ion conductive solid electrolyte 10 isinterposed between the oxide sensing electrodes 20.

Preferably, the oxygen ion conductive solid electrolyte 10 is formedfrom one of the selected from; stabilized zirconia, CeO₂ or ThO₂, andthe oxide sensing electrode 20 is formed from one or more oxidesselected from NiO, CuO, NiO—YSZ, LaCoO₃, ZnO or 2CuO.Cr₂O₃, and thenoble metal electrode 30 is formed of platinum or gold.

Further, the present invention provides a calculating method of totalNO_(x) concentration, comprising the steps of a) measuring voltages ofeach of two or more NO_(x) sensors 100 as described above; b)substituting the measured voltages into a series of NO_(x) concentrationcalculating formular of each of the NO_(x) sensors 100 so as tocalculate NO and NO₂ concentrations separately; and c) adding the NO andNO₂ concentrations so as to calculate the total NO_(x) concentration.

For example, the NO_(x) concentration calculating formular has a form ofV=a₁lnP_(NO2)−a₂P_(NO)+a₃, and coefficients of the NO_(x) concentrationcalculating formular is changed according to the forming materials andthe process of the oxide sensing electrode 20 and currents applied tothe NO_(x) sensor 100.

Furthermore, the present invention provides a calculating method oftotal NO_(x) concentration, comprising the steps of a) measuringcurrents from two or more pairs of interfaces of a NO_(x) sensor 100 asdescribed above; b) substituting the measured currents into a series ofNO_(x) concentration calculating formulars of each of the NO_(x) sensors100 so as to calculate NO and NO₂ concentrations separately; and c)adding the NO and NO₂ concentrations so as to calculate the total NO_(x)concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic view of a conventional mixed potential typeNO_(x) sensor.

FIG. 1 b is a graph showing electromotive force of the NO_(x) sensor ofFIG. 1 a in case that NO or NO₂ accounts for a major percent.

FIG. 1 c a graph showing the electromotive force in case that NiO as asensing electrode is formed for the NO_(x) sensor of FIG. 1 a.

FIG. 1 d is a graph showing the electromotive force in case that CuO asthe sensing electrode is formed for the NO_(x) sensor of FIG. 1 a.

FIG. 2 is a schematic view of another conventional NO_(x) sensor.

FIG. 3 a is a schematic view of a NO_(x) sensor according to the presentinvention.

FIG. 3 b is a graph showing the voltage in case that NiO and CuO assensing electrodes are respectively formed for the NO_(x) sensor of FIG.3 a.

FIG. 3 c is a graph showing the voltage in case that NiO and LaCoO₃ assensing electrodes are respectively formed for the NO_(x) sensor of FIG.3 a.

FIG. 4 is a schematic view of another NO_(x) sensor according to thepresent invention.

FIG. 5 is a schematic view of yet another NO_(x) sensor according to thepresent invention.

FIG. 6 a is a schematic view of yet another NO_(x) sensor according tothe present invention.

FIG. 6 b is a graph showing the voltage in case that NiO—YSZ, NiO, CuO,2CuO.Cr₂O₃ as sensing electrodes are respectively formed for the NO_(x)sensor of FIG. 6 a.

FIG. 7 a is a schematic view of yet another NO_(x) sensor according tothe present invention.

FIG. 7 b is a graph showing the voltage in case that NiO—YSZ as asensing electrode is formed for the NO_(x) sensor of FIG. 7 a.

FIG. 8 is a flow chart showing a calculating method of total NO_(x)concentration.

DETAILED DESCRIPTION OF MAIN ELEMENTS

100: NO_(x) sensor

10: oxygen ion conductive solid electrolyte

20, 20-1, 20-2, 20-3, 20-4: oxide sensing electrode

30: noble metal electrode

40: lead line

Sa˜Sc: steps for calculating total NO_(x) concentration

BEST MODE FOR CARRYING OUT THE INVENTION

Practical and presently preferred embodiments of the present inventionare illustrative as shown in the following Examples and ComparativeExamples.

A NO_(x) sensor of the present invention includes an oxygen ionconductive solid electrolyte 10; an oxide sensing electrode 20 formed atthe oxygen ion conductive solid electrolyte 10; a noble metal electrode30; and a lead line 40 connected to each of the oxygen ion conductivesolid electrolyte 10 or the oxide sensing electrode 20 or the noblemetal electrode 30. The oxygen ion conductive solid electrolyte 10 andthe oxide sensing electrode 20 form at least two interfaces.

The oxygen ion conductive solid electrolyte 10 is formed from one of theselected from stabilized zirconia, CeO₂ or ThO₂, and the oxide sensingelectrode 20 is formed from one or more mixtures selected from NiO, CuO,ZnO or NiO—YSZ, or one or more oxides selected from LaCoO₃ or2CuO.Cr₂O₃, and if two or more oxygen ion conductive solid electrolytes10 and oxide sensing electrodes 20 are formed, they may be respectivelyformed of the same material or other materials.

In the NO_(x) sensor 100 of the present invention, the oxygen ionconductive solid electrolyte 10 and the oxide sensing electrode 20 maybe formed into various shapes so as to have two or more interfaces. Itwill be explained below with the help of the schematic drawings for anexamples, but the NO_(x) sensor 100 of the present invention are notlimited to their specific structures.

FIG. 3 a is a schematic view of the NO_(x) sensor 100 according to thepresent invention, wherein the oxide sensing electrode 20 is formed ateach of upper and lower surfaces of the oxygen ion conductive solidelectrolytes 10, and the two oxide sensing electrodes 20 arerespectively formed with different materials.

According to the NO_(x) sensor 100 as described above, if an electricalbias is applied to the oxide sensing electrode 20, reactivity to NO₂ isenhanced at the negative electrode, while reactivity to NO is enhancedat the positive electrode.

FIG. 3 b is a graph showing the voltage of the NO_(x) sensor 100 of FIG.3 a in case that the negative electrode is formed of NiO and thepositive electrode is formed of CuO and currents of 5 μA is applied at atemperature of 700□ and an oxygen partial pressure of 10%, and FIG. 3 cis a graph showing the voltage obtained from the NO_(x) sensor 100 ofFIG. 3 a in case that the negative electrode is formed of NiO and thepositive electrode is formed of LaCoO₃ and currents of 5 μA is appliedat a temperature of 700□ and an oxygen partial pressure of 10%.

As shown in FIGS. 3 b and 3 c, it can be understood that sensitivity toNO is remarkably increased in comparison with the conventional NO_(x)sensor of FIGS. 1 c and 1 d, which has only a single interface of oxideelectrode. That is, on the basis of the graph of FIGS. 1 c and 1 d, theelectromotive force according to NO and NO₂ concentration can beexpressed by following experimental formulas (1) and (2):

EMF=0.03635lnP _(NO2)−72.31P _(NO)+0.3829   (1)

EMF=0.02866lnP _(NO2)−37.40P _(NO)+0.2941   (2)

Further, on the basis of the graph of FIGS. 3 b and 3 c, the voltageaccording to NO and NO₂ concentration can be expressed by followingexperimental formulas (3) and (4):

V=0.07228lnP _(NO2)−192.9P _(NO)+0.6471   (3)

V=0.06427lnP _(NO2)−91.29P _(NO)+0.4355   (4)

Comparing with the formulas (1) and (2) and the formulas (3) and (4), itcan be understood that a coefficient with respect to NO concentration(P_(NO)) in of FIGS. 3 b and 3 c is considerably increased compared witha coefficient with respect to NO concentration (P_(NO)) in of FIGS. 1 cand 1 d.

Substantially, in the NO_(x) sensor 100 according to the presentinvention, as shown in FIG. 3 a, the sensitivity to NO₂ is increasedtwice or more and the sensitivity to NO is increased three times or moreas compared with the convention NO_(x) sensor of FIG. 1 a. Therefore,the NO_(x) sensor 100 of the present invention can precisely measure anamount of NO_(x) in case that the NO or NO₂ accounts for a majorpercent.

FIG. 4 is a schematic view of another NO_(x) sensor 100 according to thepresent invention. The NO_(x) sensor 100 of FIG. 4 has the samestructure as that of FIG. 3 a, but the oxide sensing electrodes 20-1formed at the upper and lower surfaces of the oxygen ion conductivesolid electrolyte 10 are made of the same material.

In the NO_(x) sensor 100 of FIG. 4 in which the same oxide sensingelectrode 20 is formed at the upper and lower surfaces thereof, if anelectrical bias is applied, reactivity to NO₂ is enhanced at thenegative electrode, and reactivity to NO is enhanced at the positiveelectrode.

FIG. 5 is a schematic view of yet another NO_(x) sensor 100 according tothe present invention, wherein two oxygen ion conductive solidelectrolytes 10 are provided and an oxide sensing electrode 20 isinterposed between the oxygen ion conductive solid electrolytes 10 so asto form two interfaces.

In the NO_(x) sensor 100 of FIG. 5, if an electrical bias is applied tothe interfaces formed between the upper and lower oxygen ion conductivesolid electrolytes 10 and the oxide sensing electrode 20, reactivity toNO₂ is enhanced at the negative electrode, and reactivity to NO isenhanced at the positive electrode.

Besides the NO_(x) sensor 100 of FIG. 5, the present invention may beconstructed so that two or more oxygen ion conductive solid electrolytes10 are formed to be apart from each other at a predetermined distanceand the oxide sensing electrode 20 is interposed between the two oxygenion conductive solid electrolytes 10.

Further, the NO_(x) sensor 100 of the present invention may beconstructed so that two or more oxide sensing electrodes 20 are formedto be apart from each other at a predetermined distance and the oxygenion conductive solid electrolyte 10 is interposed between the two oxidesensing electrodes 20.

FIG. 6 a is a schematic view of yet another NO_(x) sensor 100 accordingto the present invention, wherein four oxide sensing electrodes 20-1,20-2, 20-3 and 20-4 which are respectively formed of different materialsare provided to be apart from each other at regular intervals and theoxygen ion conductive solid electrolyte 10 is interposed between theoxide sensing electrodes(20-1, 20-2, 20-3, 20-4).

FIG. 6 b is a graph showing the voltage in case that NiO—YSZ, NiO, CuO,2CuO.Cr₂O₃ as sensing electrodes are respectively formed in the NO_(x)sensor 100 of FIG. 6 a, wherein the oxide sensing electrodes 20-1, 20-2,20-3 and 20-4 of FIG. 6 a are formed, in turn, of NiO—YSZ, NiO, CuO,2CuO.Cr₂O₃ and currents of 1 μA is applied at a temperature of 700° C.and an oxygen partial pressure of 10%.

As shown in FIG. 6 b, the NO_(x) sensor 100 of the present invention hasimproved sensitivity to NO even in an atmosphere that the NO accountsfor a major percent and thus has improved performance.

Furthermore, the NO_(x) sensor 100 of the present invention ischaracterized in that two or more oxide sensing electrodes 20 are formedon one surface of the oxygen ion conductive solid electrolyte 10.

FIG. 7 a is a schematic view of yet another NO_(x) sensor 100 accordingto the present invention, wherein two oxide sensing electrodes 20 areformed on one surface of the oxygen ion conductive solid electrolyte 10.

FIG. 7 b is a graph showing voltage in case that NiO—YSZ as a sensingelectrode is formed in the NO_(x) sensor 100 of FIG. 7 a, wherein twosensing electrodes are formed of NiO—YSZ and currents of 10 μA isapplied at a temperature of 700° C. and an oxygen partial pressure of10%.

In other words, the NO_(x) sensor 100 of the present invention may beconstructed in various ways so that two or more interfaces between theoxygen ion conductive solid electrolyte 10 and the oxide sensingelectrode 20 are formed so as to increase the sensitivity to NO, therebyprecisely measuring total NO_(x) concentration.

Meanwhile, a calculating method of total NO_(x) concentration using theNO_(x) sensor 100 of the present invention can be classified into amethod using two sensors, and a method using a senser formed two or morepairs of oxide sensing electrode-electrolyte interfaces.

FIG. 8 is a flow chart showing a calculating method of total NO_(x)concentration. The calculating method of total NO_(x) concentrationusing the NO_(x) sensor of the present invention includes the steps of:a) measuring voltages or currents (Sa); b) calculating NO and NO₂concentration (Sb); and c) calculating total NO_(x) concentration.

In the voltages or currents measuring step (Sa), voltages or currentsmay respectively measured using the two NO_(x) sensors 100 of thepresent invention, or measured from two or more pairs of interfaceswithin one NO_(x) sensor.

That is, the calculating method of total NO_(x) concentration using theNO_(x) sensor of the present invention can be classified into two typesaccording to voltages or currents measuring method.

In the NO and NO₂ concentration calculating step (Sb), voltages orcurrents measured in the voltages or currents measuring step (Sa) issubstituted into a NO_(x) concentration calculating formular, and thusNO and NO₂ concentration can be calculated.

The NO_(x) concentration calculating formular may be varied according tothe materials which form the oxide sensing electrode 20, and the NO_(x)concentration calculating formular can be expressed as follows:

V=a ₁lnP _(NO2) −a ₂ P _(NO) +a ₃

The coefficients of the NO_(x) concentration calculating formular ischanged according to the forming material and process of the oxidesensing electrode 20 and the current applied to the NO_(x) sensor 100.In case that the oxide sensing electrode 20 of the NO_(x) sensor 100 isformed of NiO and CuO and currents of 5 μA is applied to the NO_(x)sensor 100, the values of a₁, a₂ and a₃ in the NO_(x) concentrationcalculating formular are respectively 0.07228, −192.9 and 0.6471. And incase that the oxide sensing electrode 20 of the NO_(x) sensor 100 isformed of NiO and LaCoO₃, the values of a₁, a₂ and a₃ in the NO_(x)concentration calculating formular are respectively 0.06427, −91.29 and0.4355. In case that the oxide sensing electrode 20 of the NO_(x) sensor100 is formed of NiO and CuO, the NO_(x) concentration calculatingformular of the NO_(x) sensor 100 is the same as the formular (3), andin case that the oxide sensing electrode 20 of the NO_(x) sensor 100 isformed of NiO and LaCoO₃, the NO_(x) concentration calculating formularof the NO_(x) sensor 100 is the same as the formular (4).

V=0.07228lnP _(NO2)−192.9P _(NO)+0.6471   (3)

V=0.06427lnP _(NO2)−91.29P _(NO)+0.4355   (4)

That is, the NO and NO₂ concentration calculating step (Sb) is tocalculate each of the NO and NO₂ concentrations by substituting themeasured voltage into the NO_(x) concentration calculating formular.

In the total NO_(x) concentration calculating step (Sc), the NO and NO₂concentrations which are calculated in the NO and NO₂ concentrationcalculating step (Sb) are added so as to calculate the total NO_(x)concentration.

According to the present invention as described above, since thesensitivity to NO can be increased, it is possible to precisely measurethe NO_(x) concentration. Also it is possible to facilely calculate NO,NO₂ and NO_(x) concentration using a simple method.

INDUSTRIAL APPLICABILITY

According to the NO_(x) sensor and the calculating method of measuringtotal NO_(x) concentration using the same, it is possible to prevent thedeterioration of the sensitivity even in an atmosphere that the NOaccounts for a major percent, thereby increasing the measurementprecision. And by using a simple method in which the voltages orcurrents measured from two or more sensors or a sensor having two ormore oxide electrode-electrolyte interfaces is substituted into theNO_(x) concentration calculating formular, it is possible to facilelycalculate the NO, NO₂ and NO_(x) concentration.

Those skilled in the art will appreciate that the conceptions andspecific embodiments disclosed in the foregoing description may bereadily utilized as a basis for modifying or designing other embodimentsfor carrying out the same purposes of the present invention. Thoseskilled in the art will also appreciate that such equivalent embodimentsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

1. A NO_(x) sensor, comprising: an oxygen ion conductive solidelectrolyte 10; an oxide sensing electrode 20 formed at the oxygen ionconductive solid electrolyte 10; a noble metal electrode 30; and a leadline 40 connected to each of the oxygen ion conductive solid electrolyte10 or the oxide sensing electrode 20 or the noble metal electrode 30,wherein there have at least more than two interfaces of between oxygenion conductive solid electrolyte 10 and the oxide sensing electrode 20to form a closed electric circuit to pass currents or apply voltagethrough the connecting lead line
 40. 2. The NO_(x) sensor as set forthin claim 1, wherein NO_(x) concentration is obtained by voltagesmeasured after a constant currents is applied between the two lead linesthrough which more than two interfaces of the oxide sensing electrodes20 are involved in the electric circuit or by currents measured after aconstant voltages is applied between the two lead lines through whichmore than two interfaces of the oxide sensing electrodes 20 are involvedin the electric circuit.
 3. The NO_(x) sensor as set forth in claim 1,wherein at least two or more oxide sensing electrodes 20 are formed on asurface of the oxygen ion conductive solid electrolyte
 10. 4. The NO_(x)sensor as set forth in claim 1, wherein more than or equal to one oxidesensing electrode 20 are formed on upper and lower surfaces of theoxygen ion conductive solid electrolyte
 10. 5. The NO_(x) sensor as setforth in claim 1, wherein two or more oxygen ion conductive solidelectrolytes 10 are formed to be apart from each other at apredetermined distance, and the oxide sensing electrode 20 is interposedbetween the oxygen ion conductive solid electrolytes
 10. 6. The NO_(x)sensor as set forth in claim 1, wherein two or more oxide sensingelectrodes 20 are formed to be apart from each other at a predetermineddistance, and the oxygen ion conductive solid electrolyte 10 isinterposed between the oxide sensing electrodes
 20. 7. The NO_(x) sensoras set forth in claim 1, wherein the oxygen ion conductive solidelectrolyte 10 is formed from one of the selected from; stabilizedzirconia, CeO₂ or ThO₂.
 8. The NO_(x) sensor as set forth in claim 7,wherein the oxide sensing electrode 20 is formed from one or more oxidesselected from NiO, CuO, NiO—YSZ, LaCoO₃, ZnOor 2CuO.Cr₂O₃.
 9. The NO_(x)sensor as set forth in claim 8, wherein the noble metal electrode 30 isformed of platinum or gold.
 10. A calculating method of total NO_(x)concentration, comprising the steps of: a) measuring voltages orcurrents of each of two or more NO_(x) sensors 100 according to claim 1;b) substituting the measured voltages or currents into a series ofNO_(x) concentration calculating formulars of each of the NO_(x) sensors100 so as to calculate NO and NO₂ concentrations separately; and c)adding the NO and NO₂ concentrations so as to calculate the total NO_(x)concentration.
 11. The calculating method of total NO_(x) concentrationas set forth in claim 10, wherein the NO_(x) concentration calculatingformular has a form of V=a₁lnP_(NO2)−a₂P_(NO)+a₃, where a₁, a₂, and a₃are constants in case that we measure voltage.
 12. The calculatingmethod of total NO_(x) concentration as set forth in claim 11, whereincoefficients of the NO_(x) concentration calculating formular is changedaccording to the forming materials and the process of the oxide sensingelectrode 20 or currents applied to the NO_(x) sensor
 100. 13. Acalculating method of total NO_(x) concentration, comprising the stepsof: a) measuring each voltages or currents from two or more pairs ofinterfaces of a NO_(x) sensor 100 according to claim 1; b) substitutingthe measured voltages or currents into a series of NO_(x) concentrationcalculating formulars of each of the NO_(x) sensors 100 so as tocalculate NO and NO₂ concentrations separately; and c) adding the NO andNO₂ concentrations so as to calculate the total NO_(x) concentration.